Substrates which are fabricated in an industrial setting are usually subjected to a variety of processing steps. For example, metal substrates, after being cast, may undergo many procedures to achieve a final product, such as grinding, cold-working, cleaning, annealing, grit-blasting, further cleaning, and the like. There may be a variety of designed features on or in the substrate which are incorporated early on in processing, and which must be preserved through all subsequent fabrication steps.
Turbine engines provide a good illustration. The "substrate" may be a turbine blade, or may be a combustion chamber (combustor), for example. The parts are often made from high temperature metallic alloys, often referred to in the art as "superalloys". When turbines are used on aircraft, they are typically run at a temperature as high as possible, for maximum operating efficiency. Since high temperatures can damage the alloys used in the engine, a variety of approaches have been used to raise the operating temperature of the metal components. One approach calls for the incorporation of internal cooling channels in the component, through which cool air is forced during engine operation. Thus, the "designed feature" in this instance is a pattern of cooling holes which extend from one surface of the part to another. For example, the holes may extend from a cooler surface of a combustor to a "hot" surface which is exposed to combustion temperatures of at least about 1200 C. The cooling holes are usually formed in the substrate by specialized laser-drilling techniques. Cooling air (usually provided by the engine's compressor) is fed through the holes from the cooler side to the hot side of the combustor wall. As long as the holes remain clear, the rushing air will assist in lowering the temperature of the hot metal surface and preventing melting or other degradation of the component.
Another technique for protecting the metal parts and effectively raising the practical operating temperature of an aircraft engine involves the use of a thermal barrier coating (TBC). The TBC is usually ceramic-based. TBC systems frequently also include a bond coat which is placed between the ceramic coating and the substrate to improve adhesion. The use of TBC's in conjunction with the battery of cooling holes is sometimes the most effective means for protecting an engine part. However, incorporation of both systems can be very difficult. For example, the cooling holes sometimes cannot be formed in the engine part after a TBC has been applied, since lasers usually cannot effectively penetrate both the ceramic material and the metal to form the pattern of holes.
If the cooling holes are formed prior to the application of the TBC system, they may become covered and at least partially obstructed when the TBC is applied. Complete removal of the ceramic-metal material from the holes can be very time-consuming and ineffective, if not impossible. Any obstruction of the holes during engine operation can interfere with the passage of cooling air, can waste compressor power, and can possibly lead to engine component damage due to overheating.
Even if a type of laser could satisfactorily penetrate the TBC, registration and alignment difficulties would remain. For example, there would be no suitable technique for ensuring that the hole being drilled through the TBC is properly aligned with the hole previously drilled in the substrate itself.
From this discussion, one can readily understand that new methods for protecting certain features on metal substrates during subsequent processing steps would be welcome in industry. Of particular interest in the area of turbine engines would be new methods for providing open holes which communicate through various coating layers on engine parts.
These new methods should protect the designed features, but should also allow the particular features to become fully exposed after the other processing steps are complete. Furthermore, the techniques should also be useful for repairing TBC systems while retaining open holes axially aligned with previously-formed open holes in the substrate.
Moreover, the techniques involved should be completely compatible with the other processing steps, and should not adversely affect the substrate. For example, the strength and integrity of a turbine engine part should be completely retained after the treatment to protect the cooling holes has been completed.